1. Field of the Invention
This invention relates to composites of paper and plastic film, to corrugated paperboard incorporating such a composite as a liner adhered to the corrugated medium, and to methods of making the composites and the corrugated paperboard. A corrugated paperboard incorporating this composite has a liner component is especially useful as a sheet on which bakery goods are baked. It has been found that this composite liner will withstand temperatures in an oven up to 350xc2x0 F. for a period of forty to eighty minutes even though the internal temperature of the bakery product will reach an internal temperature of 200-208xc2x0 F. This composite liner is especially useful because of the excellent release characteristics from the baked product following baking. Use of the composite eliminates the need for and reduces the requirement for the use of grease in the baking process. The bakery product can be baked upon a composite sheet or in a tray made from the corrugated paperboard incorporating the composite as a liner. The invention is also useful in the packaging field in the form of containers, but it finds advantageous utility in the fabrication of other products, such as point-of-purchase displays, wall panels, posters, and the like, where a combination of the unique structural features of the invention and high quality graphics is desired. The invention is particularly felt to satisfy a long-felt need in the area of folding cartons for packaging, whether as primary packaging or secondary packaging, in composite form or as a liner for corrugated paperboard. The moisture barrier characteristics of the composite liner board permits the construction of cartons that can be used to keep a product cold with ice which is retained within the carton as it melts. Corrugated paperboard having a composite liner board component can also be utilized for keeping a product hot for a period of time because of the resistance of the composite to heat and its ability to retain heat within the carton. Although the composite of paper and plastic film is highly useful and advantageous in itself, it is a particularly important characteristic of the composite that it can withstand the conditions in the xe2x80x9cdouble backerxe2x80x9d part of a conventional corrugating machine such that it can be combined with a single faced corrugated board to form a double faced corrugated board without material deterioration of the composite which, to my knowledge, has previously been considered impossible as a practical matter. Important characteristics of the composites alone or as incorporated in corrugated paperboard include scuff and scratch resistance, strength, stretch resistance, tear resistance, separation resistance, resistance to xe2x80x9ccheckingxe2x80x9d (cracking in a score line) in the forming of blanks for folding cartons, surface smoothness and gloss, graphics quality and durability, fragrance barrier quality, and moisture barrier quality.
2. Background
It is known in the packaging and other arts to form packaging material in the form of a laminate of paper and plastic film secured together by an. adhesive. For instance, particularly in the field of flexible packaging, various films have been laminated to paper with various adhesives, of which polyethylene is one example. For instance, I am aware that a flexible packaging material has been made by laminating xe2x80x9cMylarxe2x80x9d polyester film (a product of E. I. DuPont DeNemours and Co. Inc., Wilmington, Del., U.S.A.) to thin high-quality paper using a polyethylene adhesive. I am also aware of Peer U.S. Pat. No. 4,254,173 issued Mar. 3, 1981, which proposes a secondary container packaging material (e.g., a six-pack can or bottle wrap) comprising a paper material laminated to a plastic film. The patent discloses various films, including polyester, various papers such as Kraft paper of various weights, and various adhesives, including polyethylene, to bond the film to the paper. It is disclosed in this patent and generally known otherwise that the film can be reverse printed with graphics prior to lamination. To my knowledge, all prior proposals have used the adhering agent only as an adhesive and, thus, have used only that amount necessary to achieve adhesion of the two laminated components. I do not find in the prior art any recognition or appreciation of the possibility of using the adhering agent not just as an adhesive, but rather as an impregnant and surface enhancer so as to achieve the characteristics and advantages of the present invention.
It is a basic purpose of the present invention to provide an improved composite of paper or paperboard and plastic film in which the adhering agent is used not merely as an adhesive, but rather as an impregnating and bonding agent and as an enhancer of the paper surface by essentially forming a new surface above the paper or paperboard. By operating under conditions which ensure substantial impregnation of the adhering agent into the paper, but with a substantial part overlying the surface of the paper and firmly adhering to the plastic film, I obtain a product which is highly resistant to separation, which enhances the graphics quality of reverse printed film, which will withstand the rigors of the double-backer portion of a conventional corrugating machine, which minimizes the normal adverse effects of scoring, cutting and folding, in the formation of a carton, and which generally is a new and improved product capable of many uses.
It is a further purpose of this invention to develop a composite paperboard material that can be used as a sheet for baking bakery goods which would eliminate or reduce the need for grease as a release agent for bakery goods after baking. It is a further purpose to develop a composite paperboard with good moisture resistant characteristics for use in baking products or in keeping products cool within a carton made of the composite paperboard.
In making a composite of paper and plastic film in accordance with my invention, a web of paper and a web of plastic film are passed into and through the nip of a pair of nip rolls, and a layer of molten polymer-impregnating and bonding agent is extruded into the nip between the webs on the entry side of the nip. One or more of the speed of the webs, the temperature of the molten polymer, the pressure or spacing of the rolls at the nip, and the rate of extrusion is or are controlled relative to the porosity and surface characteristics of the paper web such that a substantial portion of the molten polymer impregnates partially into and becomes part of the paper web, and a substantial portion lies outwardly of the surface of the paper web and solidifies to form an enhanced surface. This surface is essentially a new surface, to which the film is bonded and on which it is supported clear of the paper surface. There is collected from the nip rolls a composite comprising paper partially impregnated with solidified polymer, a contiguous layer of solidified polymer having the new surface outwardly of the paper surface, and the plastic film bonded to the new surface. In the preferred practice of the invention, the inner surface of the film is reverse printed with desired graphics by any of several printing processes. Preferably the molten polymer comprises polyethylene, the film is bioriented, and the paper is Kraft paper. The molted polymer may be a copolymer of ethylene and acrylic acid. A particularly preferred film is bioriented polyester. Adherence of the components in the resulting composite is such that peeling essentially is not possible, in that the fibers of the paper will separate before the solidified polymer or the film will peel from the paper. As mentioned above, one of the most important and, indeed, surprising features of the composite is that it can be passed through the double-backer portion of a conventional corrugating machine to form the outside liner of a double-faced corrugated paperboard, which typically involves movement of the exposed face of the film along a hot plate section maintained at temperatures up to about 350xc2x0 F. Equally as important and surprising is the ability of either the composite or the corrugated paperboard of which it forms the outer liner surface to withstand the rigors of formation into folding carton blanks and folded cartons with little or no separation, xe2x80x9ccheckingxe2x80x9d, etc. The graphics quality which can be obtained and maintained in the end product is believed to be decidedly superior.
While the foregoing sets forth some of the basic features and aspects of the invention, subsidiary features and aspects of varying degrees of importance will be brought out in or apparent from the ensuing description and illustration of preferred embodiments.